The present invention relates to non-linear control loop circuits and more particularly to a high speed automatic gain control loop for attenuation control.
Automatic gain control circuits (AGC) are well known in the prior art and are generally used to maintain a predetermined signal level at an output in spite of variations in the signal level at an input. Such circuits are used in a wide variety of electronic devices including radio and television receivers and other communication systems. One example of an automatic gain control circuit includes the use of an attenuator in an automatic gain controlled IF amplifier portion of a signal receiving circuit. In such systems, the attenuator operates to provide a relatively constant radio frequency (RF) signal output for a varying RF signal input by inserting or removing an impedance between the RF input and RF output.
In the above prior-known systems, most RF attenuators have an attenuation versus control signal (voltage or current) characteristic which is non-linear. This means that the voltage or current used to control the magnitude of attenuation is not linearly related to the magnitude of attenuation achieved. This is particularly true for PIN diode attenuators and other similarly constructed attenuators. Typically, PIN diode attenuators may be controlled by sensing the output of the attenuator with a conventional detector circuit, integrating the detected output, and driving a control line on the attenuator to increase or decrease the attenuation in response to the RF output.
When using attenuators or other control circuits having non-linear control signal versus output characteristics, the detected RF output signal normally causes a high gain in the feedback loop in the high attenuation region of operation and a low gain in the low attenuation region of operation. Since it is known that high loop gain enables a faster control response, ano since fast response is often desired in the electronic circuits involved, it is necessary to fix loop gain as high as possible while still providing loop stability. However, when prior art AGC circuits are constructed to be stable in the high attenuation regions, then the loop gain will decrease and respond slowly in the low attenuation region due to the non-linear relationship between the control signal and attenuation produced. Conversely, if the feedback loop gain is increased to speed up loop operation in the low attenuation region, then in the high attenuation region, the loop will be driven to an unstable condition, again as a result of the non-linear relationship between the control signal and attenuation produced.
In view of the above problems, attempts have been made to reduce the non-linear relationship between the control signal and attenuation response. In order to accomplish this, many prior art solutions have proposed the use of a linearizing circuit placed between the integrator and the control line of the attenuator. The linearizer essentially acts as a driver circuit which responds to the detected RF output so that the control signal achieves a linear relationship with attenuation over the range of operation of the attenuator. One of the disadvantages of this technique, however is the resulting complexity of the linearizer which is more expensive, requires more elements, is increased in size, has a higher repair rate and expends more power. In high technology electronic systems, such characteristics are unacceptable, particularly where there is great emphasis to reduce size, weight, repair rate, power requirements and expense of state-of-the-art devices.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an automatic gain control circuit which will provide greater stability and improved response time over its range of operation.